sched/walt: include missing header for arm_timer_read_counter()

Include clocksource/arm_arch_timer.h to fix implicit function
declaration of ‘arch_timer_read_counter’ build error for ARCH=arm.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
[jstultz: Cherry-picked from common/android-3.18]
Signed-off-by: John Stultz <john.stultz@linaro.org>